1. Field of the Invention
The present invention relates to the art of gas flow rate calibrations and, in particular, to flow rate calibration benches and methods for accurately measuring and establishing the flow rate of a gas for use as a calibration standard for flow meters and other applications.
2. Prior Art
Because flow measurement is fundamental to many industrial and scientific processes, flow meters are applied to processes which extend over extremely wide ranges of flow rates. To maintain accuracy of this range of application requires that calibration systems capable of operating on the same range of flow rates and gases as do the flow rate meters themselves be readily available and efficiently operable.
Fluid flow rate meters are well known to the prior art. In the most general description, these devices measure fluid flow by sensing and quantifying selected physical parameters of the fluid flowing in a given flow path.
Primary flow calibration systems must allow users to verify by reference to primary standards, of length and time, the accuracy of the system, and thereby to calibrate mass flow meters and flow controllers automatically with the highest degree of accuracy.
One class of such devices has been established in the prior art.
In such devices the gas enters a precision-bored glass tube in which a mercury-sealed piston is located. As gas enters the tube, with the volume below the bottom of the piston, the sealed piston rises. The diameter, and therefore the cross sectional area of the tube is known in precise terms. This means that the vertical distance swept out by the piston in a given time is the volume displaced by the piston over that period of time.
Done manually, this method is true, but slow, and subject to human error, because of the requirement that a technician on hand with a stop watch, carefully note the position of the piston and start and stop the timer at the exact moment of passage by reference points.
The manual method has been generally replaced with electronic systems that measure piston movement precisely and which can resolve time into small parts of a second, and distance into small fractions of a centimeter.
Operating over a wide range of flow presents problems to the flow meter and calibration standards alike.
Several attempts at solving the problem of calibration of the primary flow rate have been attempted. Porter, U.S. Pat. No. 3,125,879, uses photocells to detect the passage of the piston through a predetermined range. The photocells use reflected light from the ring of mercury which forms the seal between the piston and the tube in which the piston moves. This method of position detection suffers from the fact that the mercury ring does not present a sharp edge, nor do the sensors have a narrow enough range of perception of the reflected light. In addition, the index of refraction of the glass causes uncertainty in the position of the piston with respect to its perceived reflection at the sensor.
The Porter class of devices also suffers from the fact that only fixed spacings are used for the measurement of volume. The area swept by the piston, as it goes through the fixed area between two points, does indicate an accurate measure of the volume and by measuring the time of the piston travel, the rate of flow is determined on the average over that interval. However, at any given period within the interval, the flow rate may vary widely without it being known.
Another method of measuring flow rates in the prior art employs a band which attaches to the piston and which passes over a precision pulley which drives the shaft of an optical encoder. Jackson, U.S. Pat. No. 4,307,601, is typical of this type of device.
The Jackson device improves over Porter in that it is possible to tell more quickly the amount of volume over an increment of time, and the piston does not have to fall through the entire length of the chamber between two fixed intervals. However, there are problems in using any kind of noncontinuous resolution encoder. Even the Jackson device which is described as having an encoder having 2000 counts per revolution would suffer from the inaccuracy which implies; For the dimensions listed Jackson would yield an inaccuracy of 0.003 inch per count merely because of the finite encoder steps. This is a fundamental limitation on the accuracy of the device. In addition, other inaccuracies may be the result of slippage of the band over the pulley, and the pulley's eccentricities. All of these sources of error must be taken into account.
Neither of the devices described above, but particularly Porter, can accurately determine whether the piston is rising at a constant rate, an important parameter if the flow instantaneous rate rather than merely the accuracy is to be determined accurately.